This invention is related to image processing, and in particular to a method to correct for artifacts related to having a beam-splitter in the image path, and to a method to correct for multiple images—so called ghost images—that may appear as a result of the beam-splitter.
Electronic image display devices that use a beam splitter are known and commonly used. For example, common LCD projection displays use one or more beam splitters, as do projection displays that provide a viewer with a 3D image that has depth, typically aimed at three-dimensional (3D) telepresence systems.
Beam splitters typically use a partially-silvered glass element so that one image is transmitted through the glass, and another is reflected by the partially-silvered surface, typically the front surface.
With such a device, there is a chance that there is also reflection by the back surface of the glass. Coatings are commonly used to minimize such reflections. However, such multiple image artifacts, also called “ghost image” artifacts still may appear.
Thus, in beam-splitting applications, or in any application using a device that has a partially-silvered finite-thickness reflector to provide a reflection of an image for a display image, there is a chance that a secondary incidental reflection appears in the output.
FIG. 1 shows an example of how multiple image artifacts are introduced by secondary reflection in beam splitting applications that use a beam splitter or any other partially-silvered element to project a reflection of an image. FIG. 1 shows, in schematic form, an exemplary display 100 that includes a partially-silvered glass reflector 101 with a front surface 103 and a back surface 105. A digital image is converted to light in the display. For simplicity, this is shown as light image generator 107. A beam 111 of the image from light generator 107 is shown incident on the front surface. The principal intended reflection 113 in this application appears to a viewer 121 and comes from the front surface 103 of the glass. However, a secondary reflection 115 can come from the rear surface 105 side of the glass, adding a “ghost” of the displayed image on top of itself, spatially offset from the intended image. This secondary ghost image distorts the observed projected image from what was originally displayed, thereby interfering with the accuracy of the reflected image.
The following models the process. Assuming the digital content prior to the display is represented by X(i, j) as shown in FIG. 1, where i is the vertical dimension on the drawing sheet, and also the direction of the offset in the ghost reflection, and j the horizontal dimension perpendicular to the plane of the drawing. The coordinates i and j are expressed herein scaled to be in the same scale as an image appearing to the viewer 121. That is, any and all scaling, inversions, attenuation, etc., are incorporated into the representation X(i, j), so that if no ghost artifact was present, an image X(i, j) appears to the viewer 121. How to so incorporate all the scaling, inversions, attenuation, and so forth that occurs in the physical display would be clear to those in the art. As a result of the two surfaces in the reflector 101, the observer 121 sees an image that is the intended reflected image X(i, j) together with the ghost image, made fainter by a factor denoted α, and shifted by an amount denoted Δ in the i direction—what we call the “vertical” direction. Thus, the observer 121 sees an image denoted Xobserved(i, j), withXobserved(i,j)=X(i,j)+αX(i+Δ,j),
where α X(i+Δ, j) represents the “ghost” image generated by the secondary reflection, Δ represents the content shift introduced by the thickness of the glass used by the reflector, and a represents the attenuation of the secondary reflection relative to the primary reflection.
There is a need in the art for a method and for a computer program product to remove the ghost image—the α X(i+Δ, j)—so that the observer can see an intended image denoted by X(i, j).